The storage capacity of one layer of an optical disc greatly depends upon a wavelength of a used semiconductor laser and a numerical aperture (NA) of an objective lens. The shorter the wavelength of the semiconductor laser is or the larger NA is, the larger the recording density can be and the capacity per one layer can be increased. The mainstream of optical disc drives currently distributed on the market is a digital versatile disc (DVD) drive using red light having a wavelength in the vicinity of 650 nm and an objective lens the NA of which is 0.6; however, as a drive the recording density of which exceeds the recording density of the DVD drive, an optical disc drive using a semiconductor laser that emits a violet beam having a wavelength in the vicinity of 405 nm for a light source and an objective lens the NA of which is 0.85 is also shipped. For a method of further increasing currently achieved recording density, the reduction of a used wavelength is conceivable; however, it is estimated that the development of a semiconductor laser that emits a beam in an ultraviolet region the wavelength of which is shorter than that of the violet beam is difficult. Besides, as for enhancing NA of an objective lens, as a limit of the NA of the objective lens in air is 1, the enhancement depending upon the NA of the objective lens of recording density has been also difficult.
In such a situation, for a method of increasing the capacity of one optical disc, two-layer structure is adopted. On Jpn. J. Appl. Phys. Vol. 42 (2003) pp. 956 to 960, the technology of a two-layer phase change disc is described. When a laser beam is radiated onto a two-layer optical disc, it simultaneously irradiates adjacent layers and therefore, layer-to-layer crosstalk comes into question. To reduce this problem, layer-to-layer spacing is increased. As a laser beam is focused and a layer except a target layer (a corresponding layer) is off a focused spot of the layer beam, crosstalk can be reduced.
In the meantime, when layer-to-layer spacing is widened, spherical aberration comes into question. Recording layers are buried in polycarbonate the refractive index of which is different from that of air and the spherical aberrations are different depending upon depth from the surface of the disc. An objective lens is designed so that the spherical aberrations are small for the specific layers and as distance from the surface is different between focal spots when a focus of the laser beam is shifted on another layer, spherical aberration occurs. This aberration can be corrected by locating an expanding lens optical system normally including two lenses or a liquid crystal element in front of the objective lens. That is, the aberration can be corrected by changing distance between the two lenses or a phase of the liquid crystal element. However, when it is considered that a compensable range of the liquid crystal element or a mechanism for moving the lens is provided in a small-sized optical disc drive, it is difficult to correct large spherical aberration. Therefore, the thickness of the whole multi-layer disc is limited and layer-to-layer spacing is made narrow in a multi-layer optical disc having multiple layers. Therefore, in an actual optical disc drive, layer-to-layer crosstalk is left unsolved.
To reduce the crosstalk, in JP-A No. 2005-302084, it is utilized that focused spots of reflected lights from a target layer and an adjacent layer are different on an optical axis when reflected light from a multi-layer optical disc is focused by a lens. Only the target reflected light can be extracted by arranging a minute mirror on the optical axis and crosstalk can be reduced. However, as a method of bending the reflected light from the optical disc in a lateral direction of the optical axis is adopted, an optical pickup is necessarily large-sized. Besides, in JP-A No. 2002-367211, a method of removing reflected light from an adjacent layer using a critical angle prism is proposed. In this method, light at an angle equal to or exceeding a certain angle with an optical axis is removed by the critical angle prism utilizing a fact that reflected light from the adjacent layer is turned divergent light or convergent light though reflected light from a corresponding layer is turned collimated light. In this method as two critical angle prisms are used, an optical pickup is also large-sized.